To take a weighted average of all the naturally occurring isotopes of an element in order to arrive at an average atomic mass, you would
o multiply the mass of each isotope by the decimal fraction representing its abundance naturally, then add all these products
together.
O use the isotope with the largest mass.
O use the isotope with the most average mass.
o add all the masses of all of the isotopes, then divide by the number of isotopes.

o multiply the mass of each isotope by the decimal fraction representing its abundance naturally, then add all these products  together.

Explanation:

To find the average atomic mass of an atom, take the weighted average of all naturally occurring isotopes of an element and sum them together.

 Average mass of all isotopes  =  summation of all the  mass of isotopes of the element x abundance of the isotope.

 The abundance of the isotope is usually taken as a decimal fraction.